1. Field of the Invention
The present invention relates to a zoom lens, and more particularly, to a zoom lens used in an optical apparatus, such as a video camera, digital camera, or film camera.
2. Related Background Art
There has been available a so-called four-unit zoom lens consisting of lens units of positive-negative-positive-positive refractive powers as a lens system for use in an optical apparatus, such as a video camera or a digital still camera; it includes, in succession order from the object side, a first lens unit for zooming and focusing which is stationary and of positive refractive power, a second lens unit having negative refractive power and adapted to move on the optical axis to effect zooming, a third lens unit for zooming and focusing which is stationary and of positive refractive power, and a fourth lens unit of positive refractive power adapted to move on the optical axis for the purpose of correcting image plane fluctuation as a result of zooming and effecting focusing.
Japanese Patent Application Laid-Open No. 2000-121940, Japanese Patent Application Laid-Open No. 2000-121941, etc. propose a small-sized four-unit zoom lens of this type in which the first lens unit is composed of a junction lens consisting of negative and positive lenses and a positive lens.
Apart from this, four-unit zoom lenses of this type are proposed, for example, in Japanese Patent Application Laid-Open No. 4-43311 (corresponding to U.S. Pat No. 5,189,558), Japanese Patent Application Laid-Open No. 5-72472 (corresponding to U.S. Pat. No. 5,572,364), Japanese Patent Application Laid-Open No. 6-130297 (corresponding to U.S. Pat. No. 5,396,367), and Japanese Patent Application Laid-Open No. 8-304700.
Recently, there has been an increasing demand for a small and inexpensive zoom lens of high performance.
Generally speaking, in a zoom lens, by increasing the refractive powers of the lens units, the movement amount of each lens unit for achieving a predetermined zooming ratio is reduced, thereby making it possible to reduce the entire length of the lens.
However, simply increasing the refractive power of each lens unit involves an increase in fluctuation in aberration, with the result that it is difficult to obtain a satisfactory optical performance over the entire zooming range.
When adopting in a zoom lens a rear focusing system in which focusing is effected in the lens units other than the first unit, a reduction in the size of the entire lens system is achieved. On the other hand, this leads to an increase in fluctuation in aberration at the time of focusing, making it very difficult to obtain high optical performance over the entire object distance from an object at infinity to a close-range object.